As is common, rotary and linear encoders have been utilized to measure shaft rotation or linear movement through the interposition of a scale provided with opaque and transparent regions lying next to each other in a regular pattern. When the scale is passed between a light source and a detector, the output of the detector is a series of pulses or sine waves which are detected to derive either the position of the scale or angular position of a rotary shaft.
Two of the major causes of failure for such encoders is the problem of dirt or other contaminants in the light path between the light source and the detector and/or aging of the illumination source. Typically, encoder scales become so dirty in the environment in which they are employed, that the intensity of the beam from light source to detector is so reduced by a dirty scale that spurious readings result. The same failure mechanism is caused by gradual illumination aging.
This scale condition is not readily apparent until the operation of the encoder has so deteriorated that erroneous readings cause equipment malfunction. When encoders are utilized in a servo loop for instance for positioning a valve, the erratic reading may go undetected until such time as valve operation is so impaired that catastrophic results occur. It will be apparent that such a failure could be catastrophic when such a servo system is used to control fuel to a rocket engine, or when such a valve is utilized to control a chemical process. In fact, weak signals from encoders are responsible for unexplained erratic behavior sometimes attributed to other causes within the servo system.
It is therefore necessary to provide an early warning system for the malfunctioning of rotary or linear encoders in which not only is catastrophic failure of the encoder indicated as an alarm condition, but also the deterioration of the encoder output over time is detected to indicate either a predictable mean time to failure for the encoder, or to permit encoder replacement or repair prior to outright failure.
As an additional problem, for reflectance type encoders in which the light source and the detector are on the same side of the scale and light is reflected from the source to the detector, movement of the scale away from the source/detector pair results in a decreased detector output due to a portion of the light being reflected away from the detector's position. As a result, these encoders have a varying amplitude output which is difficult to process by downstream equipment.
Moreover, an aging light source can in fact simulate a dirty scale, such that the erratic readings from the encoder may be attributed to the wrong source, usually contamination.
It is therefore necessary to provide a system which if it cannot distinguish between tdim light sources or the dirty encoder scale condition, at least provides a warning and a histogram from which the cause of the encoder output degradation can be ascertained.